1. Field of the Invention
The present invention relates to deposition techniques for forming metal regions on semiconductor substrates, and more particularly to a fabrication method for the differential selective deposition of tungsten for forming tungsten contacts on an integrated circuit chip.
2. Description of the Prior Art
In prior art techniques for selective deposition of tungsten contact metal in a via for subsequent planarizing, a substitution reaction is used where W from WF.sub.6 gas substitutes for Si in exposed silicon areas to produce a thin W surface. Hydrogen is then introduced into the CVD reactor and tungsten is deposited by reaction with the WF.sub.6 source gas.
In the present invention disclosure, this technique is improved in that NF.sub.3 is then bled into the system and a plasma is struck in the reaction chamber. This produces an etching condition such that the SiO.sub.2 surface is kept clean. U.S. Pat. No. 3,785,862, issued Jan. 15, 1974 to Grill relates to a process for depositing tungsten on a substrate which comprises silicon dioxide. The process comprises the steps of heating the silicon dioxide layer in an enclosed chamber. An inert gas, such as nitrogen, and a reducing gas, such as hydrogen, are introduced into the chamber. The silicon dioxide layer is then treated with tungsten hexafluoride for a brief period, in order to etch the silicon dioxide layer and deposit a relatively thin layer of tungsten thereon. The chamber is then purged of all the unreacted hexafluoride. The inert gas flow is discontinued, and additional reducing gas and newly vaporized hexafluoride are mixed in the chamber to reduce the hexafluoride and deposit a relatively thick layer of the metal constituent thereof on the thin layer.
Treatment of the silicon oxide surface with the hexafluoride etches the oxide layer, modifying its properties such that a refractory metal layer adheres much better than without such treatment; additionally, the simultaneous reduction of a small amount of the hexafluoride controls the etching attack during the etching step, which protects the silicon dioxide over the PN junctions from further etching.
U.S. Pat. No. 4,349,408, issued Sept. 14, 1982 to Tarng et al relates to a method of depositing a refractory metal (tungsten) on a semiconductor substrate wherein a passivation material is also deposited around the periphery of the metal.
This is accomplished by depositing an oxygen doped polycrystalline silicon layer on one surface of the semiconductor substrate and then depositing a phosphorus doped polycrystalline silicon layer on the outermost surface of the first layer. Thereafter, windows are etched in the layers to expose surface areas of the substrate where the metal is to be deposited. Afterwards, the tungsten contact is deposited on the exposed surface areas of the substrate by CVD techniques.
In accordance with a CVD technique, tungsten (W) is deposited on the surface areas by placing the substrate in a CVD reactor and heating the substrate. Tungsten hexafluoride (WF.sub.6) and an inert carrier gas such as argon (Ar) or nitrogen (N.sub.2) are then fed into the reactor and the tungsten hexafluoride will react with the silicon in accordance with the following: EQU 2WF.sub.6 +3Si.fwdarw.2W.dwnarw.+3SiF.sub.4 .uparw.
The deposition of tungsten will stop after a layer of from about 500 angstroms to about 2,000 angstroms thick has been deposited. Inasmuch as a layer of about 4,000 angstroms thick is desired, the deposition process must be altered. In order to deposit the additional 2,000 angstroms to 3,500 angstroms of tungsten, the temperature is lowered. At this point, hydrogen (H.sub.2) is added to the tungsten hexafluoride and carrier gas. The tungsten hexafluoride will react with the hydrogen to deposit the desired additional tungsten in accordance with the following: EQU WF.sub.6 +3H.sub.2 .fwdarw.W.dwnarw.+6HF .uparw.